1. Field of the Invention
The present invention relates to an image forming reflecting mirror for a constant speed optical scan and an optical scanner using this reflecting mirror.
2. Description of the Related Art
In a general well-known optical scanning system, an optical scanning operation is performed by deflecting a light beam at an equal angular velocity and forming a light spot on a scanned face by an image forming optical system.
In the image forming optical system, an f.theta. lens is widely known and is used. However, a concave mirror has been recently used instead of the f.theta. lens.
However, when a light beam incident to the concave mirror is set to a parallel light beam, the length of an optical path from the concave mirror to the scanned face is increased so that it is difficult to make an optical scanner compact.
It is therefore considered that the light beam incident to the concave mirror used instead of the f.theta. lens is set to be convergent so as to reduce the length of the optical path from the concave mirror to the scanned face and make the optical scanner compact.
When a face of the concave mirror used instead of the f.theta. lens is formed by a general spherical face or a normal quadratic surface, it is possible to preferably correct field curvature. However, it is very difficult to preferably correct the linearity of an image.